Wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTS) and a plurality of subscriber units. An established harmonised cellular radio communication system, providing predominantly speech and short-data communication, is the Global System for Mobile Communications (GSM). GSM is often referred to as 2nd generation cellular technology.
An enhancement to this cellular technology, termed the General Packet Radio System (GPRS), has been developed. GPRS provides packet switched technology on GSM's switched-circuit cellular platform. A yet further enhancement to GSM that has been developed to improve system capacity can be found in the recently standardised Enhanced Data Rate for Global Evolution (EDGE) that encompasses Enhanced GPRS (EGPRS). A still yet further harmonised wireless communication system currently being defined is the universal mobile telecommunication system (UMTS). UMTS is intended to provide a harmonised standard under which cellular radio communication networks and systems will provide enhanced levels of interfacing and compatibility with many other types of communication systems and networks, including fixed communication systems such as the Internet. Due to this increased complexity, as well as the features and services that it supports, UMTS is often referred to as a third generation (3G) cellular communication technology. In UMTS subscriber units are often referred to as user equipment (UE).
In such cellular wireless communication systems, each BTS has associated with it a particular geographical coverage area (or cell). The coverage area is defined by a particular range over which the BTS can maintain acceptable communications with subscriber units operating within its serving cell. Often these cells combine to produce an extensive coverage area.
Wireless communication systems are distinguished over fixed communication systems, such as the public switched telephone network (PSTN), principally in that mobile stations/subscriber equipment move between coverage areas served by different BTS (and/or different service providers). In doing so the mobile stations/subscriber equipment encounter varying radio propagation environments. In particular, in a mobile communication context, a received signal level can vary rapidly due to multipath and fading effects.
One feature associated with most present day wireless communication systems allows the transceivers in either or both the base station and/or subscriber unit to adjust their transmission output power to take into account the geographical distance between them. The closer the subscriber unit is to the BTS's transceiver, the less power the subscriber unit and the BTS's transceiver are required to transmit, for the transmitted signal to be adequately received and decoded by the other unit. Thus, the transmit power is typically controlled, i.e. set to a level that enables the received signal to be adequately decoded, yet reduced to minimize potential radio frequency (RF) interference. This ‘power control’ feature saves battery power in the subscriber unit. Initial power settings for the subscriber unit, along with other control information, are set by the information provided on a beacon (control) physical channel for a particular cell.
Furthermore, in a number of wireless communication systems, the effect of fast fading in the communication channel is a known and generally undesirable phenomenon caused by the signal arriving at a receiver via a number of different paths. Therefore, fast power control loops are often adopted to rapidly determine and optimize the respective transmit power level. Such power control loops introduce potential instability problems into the transmitter design.
The inventors of the present invention have identified that in the field of power control techniques, and particularly at the higher power amplifier output power levels, for example >30 dBm, the closed loop system sometimes does not operate over sufficient bandwidth to track the reference signal rapidly enough, which is as a direct result of the control slope of the PA collapsing.
In the context of the present invention, the expression ‘bandwidth’, with respect to closed loop power control operation of the transmitter, encompasses a speed at which a system responds to an input perturbation.
Once the power amplifier has reacted, the reference signal has already ramped down significantly. In this case, the power amplifier output power must then drop very dramatically to compensate for the fact that the reference signal has already dropped. Invariably, in such a situation, a switching transient hit is incurred, which causes spectral degradation of the transmit signal.
As a result of these effects, critical standards' specifications are failed, such as:                (i) Power versus time (PvT) mask, or        (ii) Out-of-band spectral emission performance.        
For power amplifiers operating in a closed loop environment, a poor control slope (between the bias input and the detector output) significantly shortens the bandwidth of the system. This is particularly the case at higher output power levels and provides a sluggish performance of the power control operation. Typically, it is intended that the radio frequency (RF) power output signal tracks a raised cosine profile response on a ‘ramp-down’ operation.
However, due to the aforementioned sluggish behaviour (hereinafter referred to as a ‘dead-zone’, as illustrated in FIG. 1), of the power amplifier, particularly when operating in a ‘closed-loop’ architecture, it takes a finite time (in the order of μsec) for the output to react to the ramp-down operation. This degrades the transient behaviour at the output of the power amplifier, which, in turn, degrades the switching transient performance of the transmitter. As a result, unfavourable interference at adjacent channels occurs that fail to meet, say, the 3GPP/ETSI 05.05 specifications.
U.S. Pat. No. 6,625,227 B1, titled “Artificial ramping of transmit power for burst transmissions” describes a mechanism for looking at the symbols used in a transmission and then initiates both ramp-up and ramp-down operations based on these symbols. In particular, the mechanism proposes modifying ramp profiles to specifically improve phase transitions.
U.S. Pat. No. 6,553,212 B1, titled “Method and apparatus for improving loop stability and speed of a power control loop”, describes a mechanism for assessing a system power and determines how fast/slow the system is at that given power. The document then proposes modifying the gain within the feedback path, which effectively modifies the closed loop bandwidth.
A need therefore exists, in general, for an improved power control arrangement and method of operation, particularly in the case of a power amplifier performance operating in a closed-loop architecture at relatively high power output levels.